Some integrated circuit devices need a constant bias current, for example, in order to place a component under conditions that are advantageous for the operation thereof.
One conventional solution for setting a bias current consists in using a current mirror. However, in some applications, for example, when gate lengths must be kept to a minimum, it is not possible to use this type of mirror due to problems matching the components of the circuit.
Moreover, the bias current must be regulated. That is to say, for example, controlled by negative feedback in order to prevent variations in its value stemming from external causes and in order to ensure a precise bias current.
Bias current regulation loops BCL0, as shown, for example, by FIGS. 1 and 2, enabling a regulated current to be passed through the device have, therefore, been produced.
In FIG. 1, the regulation loop BCL0, which regulates the bias current IDC flowing through a stage to be regulated REG0, includes a transimpedance amplifier AMP0 configured to compare the bias current IDC with a reference current Iref.
It will be recalled here that a transimpedance amplifier makes it possible, in a standard and known manner, to generate on its output a voltage signal from a current signal (for example a current variation or differential) at its input.
The amplifier AMP0 generates a regulation voltage on an output node A0. Such regulation voltage controls the conduction of a transistor T1 channelling the bias current IDC across its conduction terminals.
FIG. 2 shows the integrated circuit CI from FIG. 1, and details an example of a transimpedance amplifier AMP0.
The amplifier AMP0 includes two transistors T2, T3, which are P-MOS transistors in this example, connected into a differential pair, which make it possible to compare the bias current IDC with the reference current Iref.
The ratio of the resistances of the resistors R2 and R3 makes it possible to compare values of the bias current IDC and of the reference current Iref in accordance with the same ratio.
The reference current Iref is transmitted to the drains of the transistors T2 and T3 by a current mirror MIR including three transistors T4, T5, T6, which are N-MOS transistors in this example. The transistor T6 is diode-connected, and the two transistors T4 and T5 pass a copy of the reference current Iref to each of the two transistors T2 and T3, respectively.
A regulation loop BCL0 of this kind, the implementation of which can be likened to control of the current by negative feedback, generally requires its operation to be stabilized in order to prevent detrimental resonance effects in particular.
The phase margin reflects the stability of a system, and is equal to the difference between a phase shifted by 180° and the phase of the system at the zero-gain frequency. One conventional criterion for stability is a phase margin of greater than 45°.
The conventional solution consists in providing a stabilizing capacitor C0 connected between the output node A0 and a ground terminal GND. With RA0 being the equivalent impedance on the output node A0, the capacitor C0 introduces a cut-off frequency at ½πRA0C0.
In a system including a regulation loop, instability is linked to the position of the unity-gain frequency (set by the dominant pole ½πRA0C0) with respect to the secondary poles. When the secondary poles are set, increasing C0 makes it possible to decrease the unity-gain frequency and thus to increase the phase margin.
With this kind of assembly, the value of the capacitor C0 may be very high.
By way of illustration, the size of such a capacitor C0 on its own may be greater than the area taken up by the entire remainder of the circuit shown in FIGS. 1 and 2. Furthermore, a large capacitance in a radiofrequency amplifier may also have an antenna effect that is detrimental to the correct operation of the latter.
Another way of stabilizing the regulation loop BCL0 may consist in decreasing the transconductance of the transistor T2, this moreover introducing a loss of precision, which is likewise undesirable.
Moreover, a capacitor coupled, for example, between the gate and the drain of the transistor T1 introduces a Miller effect that makes it possible to stabilize the regulation of the current in a more compact manner.
That being said, although the Miller effect makes it possible to ensure good current regulation loop stability, in particular, due to the effect of pole splitting, it introduces constraints that are incompatible with the use of the transistor T1 as a radiofrequency amplifier, since it limits the bandwidth of the transistor T1.
Therefore, there is a need to stabilize a current regulation loop without impairing its performance while not limiting the bandwidth of the stage to be regulated and in a way that minimizes the area taken up in the integrated circuit.